Okay but this is where I get confused. Why does Discord need a single database system when discord servers are independent, right?

And the volume of traffic per Discord server must be human-processable or what would the point be? A Discord server doing 800k writes per second makes no sense.

So why not a RDBMS per Discord server, and if you want to ship all that out to a warehouse for analytics you do that as a separate problem?

Or is it that spinning up a Postgres instance per Discord server ends up being significantly more expensive than these mega distributed database systems?